Sulfate uptake by Lemna can be described by a relatively simple model: The passage of sulfate across the cell membrane into the cytoplasm where most of it is metabolized to organic compounds or transported into the vacuole without major efflux back across the cell membrane. Vacuolar sulfate slowly re-emerges to be metabolized into protein amino acids. A comprehensive picture of the metabolic consequences of regulation of sulfate uptake in response to availability of sulfur sources was obtained. As sulfate in growth medium was increased, down-regulation of high affinity sulfate uptake was more than compensated for by unregulated uptake via a "non-saturating" system. Inorganic sulfate accumulated, but formation of reduced sulfur remained constant. Some L-cystine was converted to sulfate. Presence of L-cystine in the medium down-regulated high affinity sulfate uptake and decreased the rate of sulfate organification. Net results were dose-dependent accumulation of soluble cyst(e)ine and glutathionine, but not of total sulfate and soluble methionine. L-Methionine was not metabolized to cyst(e)ine or its products. Its presence in the medium led to increased total sulfur, brought about by many fold increases in soluble methionine, S-adenosylmethionine, and S-methylmethionine sulfonium. A survey of the capacity of Lemma paucicostata to take up organic compounds such as might be present in the natural environment of this plant identified eight discrete transport systems. Reciprocal inhibition studies defined the preferred substrates for these systems as follows: neutral L-Alpha-amino acids, basic amino acids, purine bases, choline, ethanolamine, tyramine, urea and aldohexoses. Each of these systems takes up its preferred substrates at high rates. The neutral amino acid systems neither transports basic amino acids nor is inhibited by these compounds. The basic amino acid system does not transport neutral amino acids but is strongly inhibited by some, but not all, of these compounds. Maintenance of these active, specific, and discrete systems in Lemna suggests they permit this plant to utilize organic compounds occurring naturally in their environment.